Dynamic speakers are well known in the art as one of acoustic transducers used to reproduce sound of music and voice. Now, conventional speaker is described as follows with reference to drawings.
A conventional speaker comprises; (a) top yoke 1, (b) magnet 2, (c) bottom yoke 3 mounted on bottom surface of magnet 2, (d) central pole 4 incorporated with bottom yoke 3, (e) frame 6, (f) voice coil 71 wound on voice coil bobbin 72, (g) damper 19 to fix external circumference of voice coil bobbin 72 on frame 6, (h) diaphragm 10 having internal circumference fixed on voice coil bobbin 72 and external circumference fixed on edge 11 (described later), (i) edge 11 having internal circumference fixed to diaphragm 10 and external circumference fixed to frame 6 and (j) central cap 8 fixed in the center of diaphragm 10.
Then, function of a speaker with above configuration is described. Current passed through voice coil 71 wound on voice coil bobbin 72 generates electro-magnetic force perpendicular to both magnetic field direction and current direction respectively according to Fleming's Law, as the direction of the current is orthogonal to magnetic field provided inside of magnetic space built by top yoke 1 and central pole 4. Subsequently, damper 19 and edge 11 hold voice coil 71 so that the voice coil and the central pole 4 have same center axis. And when diaphragm vibrates, damper 19 and edge 11 act as springs in an amplitude direction for bringing togather a midpoint in thickeness direction of top yoke 1 and a midpoint of winding width of voice coil 71. When AC current is passed through voice coil 71, voice coil bobbin 72 and diaphragm 10 vibrate being held by damper 19 and edge 11. The vibratory motion causes air vibration to produce compressional wave as an audible sound.
However, this configuration has following drawbacks.
Firstly, vibration system does not perform an ideal piston movement but causes rolling phenomena (left-to-right rocking), due to asymmetric holding strength of damper 19 or edge 11, or asymmetric back pressure occured on diaphragm 10 when a speaker is set in a box to reproduce sounds. Whole of diaphragm 10 does not move in-phase but moves reverse phase partially during a rolling phenomena, consequently disturbance occurs in a frequency characteristics of the sound pressure as shown in FIG. 15.
Secondly, harmonic distortion occurs in frequency characteristics of the sound pressure as shown curve (b) and curve (c) in FIG. 15, as damper 19 has a non-linear relation between applied force and displacement and has a hysteresis characteristics, as well.
To solve these problems, a speaker has been disclosed in Japanese Patent No.2940236 that uses shaft and bearing instead of a damper to support piston movement of up-and-down vibration. In this disclosure, however, a problem is an occurrence of noisy sound between shaft and bearing.